Photocomposition machines operate essentially on the principle of a photographic enlarger or perhaps more closely to a motion picture projector. The later comparison is more accurate in that a photocomposition machine projects selected characters in rapid sequence.
In both the still projection or enlargement and the rapid sequence projection for motion picture display, the irradiance is selected by an iris adjustment and remains fixedly thereafter for the duration of the particular application. In the case of a photographic enlargement exposure, the iris is generally not altered until the complete exposure is made and then another adjustment is made for another font source before the next exposure. In the case of motion picture projection, the change in the optical system is made only as the age of the projection light source causes the intensity of the irradiance to change. It is a compensating adjustment rather than a positive selection for delivering alteration of the irradiance.